1. Field of the Invention
The present invention relates to optical scanning apparatuses and scanning image forming lenses.
2. Discussion of the Background
The required writing density of an optical scanning device used in a recording unit of a facsimile or digital copying machine, a laser printer or other similar image forming apparatuses, has been recently greatly increased. Also, there has been a great demand recently for ensuring that the light beam spots converged on a scanned surface are greatly reduced in size and have a uniform size.
In order to obtain an excellent recorded image having a constant image resolution by use of an image forming apparatus having an optical scanning apparatus, it is important that a beam spot diameter does not vary in relation to a scanning position, i.e., that the beam spots are uniform in diameter along a scanning line. To realize such stability of beam spot diameter, an image-surface curvature in an optical system needs to be satisfactorily corrected, and various attempts to realize the satisfactory correction of the image-surface curvature have been made.
To perform a satisfactory image writing at a remarkably high writing density such as 600 dpi or 1200 dpi, a beam spot having a small beam spot diameter is necessary. To realize the small beam spot diameter satisfactorily and with good stability, a conventional geometric optical correction of image-surface curvature, optical magnification or the like is insufficient, and it is important to make sure that a wave-optical wave aberration is constant irrespective of the image height of the beam spot. More specifically, as the beam spot diameter decreases, a larger luminous-flux diameter becomes necessary, so that paraxial correction by geometrical optics is insufficient.
In the prior art, a polygon mirror functioning as a light deflecting device is sometimes tilted to achieve a desired surface tilt for effectively deflecting a luminous flux to be impinged on a surface to be scanned. However, this surface tilt of the polygon mirror causes problems with a field of curvature and beam waist of the beam spot impinged on the surface to be scanned. Prior art devices have not recognized the combined problems with field of curvature and beam waist and therefore, have not provided solutions for correcting these problems. As most, the prior art only focused on correcting the beam waist problem only at the center image height and did not recognize the necessity or the way to correct the beam waist problem for all image heights.
Also, when multiple beams are used for a light source, the image surface curvature causes uneven scanning pitches in the sub-scanning direction. That is, the scanning line intervals are not constant over an entire range of image heights, and instead, the scanning line intervals vary according to image height.
In order to overcome the problems described above, preferred embodiments of the present invention provide an optical scanning apparatus and a scanning image forming lens which correct wave aberration satisfactorily, achieve a reliably uniform and greatly reduced beam spot diameter, while achieving a desired very high writing density.
In addition, preferred embodiments of the present invention provide an optical scanning apparatus and a scanning image forming lens which correct and minimize a beam waist at all image heights along the surface to be scanned and also correct field of curvature such that a beam spot diameter is minimized and made uniform for all beam spots impinged on the surface to be scanned.
According to one preferred embodiment of the present invention, an optical scanning apparatus includes a light source for outputting light, a first lens system arranged to receive the light output from the light source and to transmit a light flux therefrom, an optical deflector arranged to receive the light flux from the first lens system and having a deflecting reflective plane to deflect the light flux from a surface therefrom and a second lens system arranged to receive the light flux deflected from the optical deflector and to condense the deflected luminous flux into an optical beam spot on a surface to be scanned so as to form images having image heights, the luminous flux condensed by the second lens system into the optical beam spot including an optical beam waist. The second lens system has a scanning and image forming element including at least one surface including a plurality of portions each having a non-arc shape in a sub-scanning direction such that at least two of the non-arc shapes are different from each other and such that an effective writing width W and a width Fs of the sub-scanned image-surface curvature located within the effective writing width satisfies the condition Fs/W less than 0.005.
In another preferred embodiment of the present invention, an optical scanning apparatus includes a light source for outputting light, a first lens system arranged to receive the light output from the light source and to transmit a light flux therefrom, an optical deflector arranged to receive the light flux from the first lens system and having a deflecting reflective plane to deflect the light flux from a surface therefrom and a second lens system arranged to receive the light flux deflected from the optical deflector and to condense the deflected luminous flux into an optical beam spot on a surface to be scanned so as to form images having image heights, the luminous flux condensed by the second lens system into the optical beam spot including an optical beam waist. The second lens system includes a scanning and image forming element including at least one surface including a plurality of portions each having a non-arc shape in a sub-scanning direction such that at least one lens surface is a sub non-circular arc surface and a shape in a sub-scanning cross section of the sub non-arc circular shape is a non-arc shape and the non-arc shape changes in accordance with a position of the sub-scanning cross section in the main scanning direction.
It should be noted that the preferred embodiments of the present invention are intended for use with apparatuses having a writing density of about 1200 dpi, 2400 dpi or greater than 2400 dpi. However, the preferred embodiments of the present invention may also be used with apparatuses having a writing density of about 300 dpi to 600 dpi.
The second lens system may preferably include either one lens element or two lens elements or more than two lens elements.
At least one surface of the lens element or lens elements of the second lens system is preferably non arc shape in a main scanning direction.
The light source in the apparatus of preferred embodiments of the present invention may be a single light beam source or a multiple beam source.